1. Field of the Invention
The present invention relates to a discharge lamp drive apparatus which drives discharge lamps used as a backlight for a liquid crystal panel, and a liquid crystal display apparatus using the same.
2. Description of the Related Art
In recent years, with an increase in size of a screen of a liquid crystal panel, a circuit scheme which drives a plurality of discharge lamps for a backlight in parallel has been used in one liquid crystal panel. As means for driving the plurality of discharge lamps in parallel, there are a scheme which connects one end side of the plurality of discharge lamps with an inverter circuit and a transformer and grounding the other end side of the same (which will be referred to as a normal drive scheme hereinafter) and a scheme which connects one end side of the plurality of discharge lamps with an inverter circuit and a transformer and connects the other end side of the same with another transformer (which will be referred to as a differential drive scheme hereinafter).
Of these two schemes, the differential drive scheme can reduce output voltages of the transformers, which enables the use of circuit components having a small withstand voltage, thereby decreasing a cost accordingly.
In such a discharge lamp drive apparatus, however, there occurs a state in which a current does not flow between a transformer and discharge lamps (which will be referred to as an open state hereinafter) in some cases because of, e.g., a contact failure of a discharge lamp electrode with respect to a connector. Such an abnormal state, in which a liquid crystal display cannot operate appropriately, must be detected. As such means, for example, Japanese Patent Application Publication No. 6-267674 and Japanese Patent Application Publication No. 2004-241136 disclose a normal drive type discharge lamp drive apparatus which is provided with a light-off detection circuit for detecting the open state.
In the discharge lamp drive apparatus disclosed in JP 6-267674 and JP 2004-241136, since the normal drive scheme is adopted, one end side of discharge lamps is grounded and has a low voltage. Therefore, whether each discharge lamp is in the open state or not can be determined by providing a resistance between the end side of the respective discharge lamps and the ground and detecting a current flowing through the resistance.
However, in case of a discharge lamp drive apparatus adopting the differential drive scheme, since transformers are connected with both ends of the discharge lamps and both the ends of the discharge lamps have a high voltage, it is impossible to take such a circuit configuration as disclosed in JP 6-267674 and JP 2004-241136 in which the resistance is provided between the discharge lamps and the ground.
On the other hand, adopting current transformers or the like to detect a current flowing through each end of the discharge lamps, which has a high voltage, presents the problem of hindering size reduction and cost reduction of the product.